Just outside town is Flambards, Cornwall's oldest theme park. Attractions include the wartime-themed Britain in the Blitz, an aviation gallery and a reconstructed Victorian village, plus various outdoor rides: there's a log flume, a roller coaster and a spiralling Skyraker among other attractions. It's hardly Disneyland, but it'll keep the kids entertained for an afternoon. Note that outside July and August, the rides only run on certain days, so it's worth checking the website in advance.
Tickets can be validated as a seven-day return pass; make sure you ask for your wristband when you arrive at the gate. There's a 10% discount for online bookings.